The use of security devices such as substrates coated with secure coatings for adhering to and for protecting banknotes, credit cards and other valuable documents is well known. Some of these security devices provide the advantage of being decorative as well. By way of example, however not limited thereto, a security thread is a strip of material placed on the surface of a banknote document or sheet such as banknote; alternatively a security thread may be serpentined or woven into the banknote paper (a window type effect) to confer additional security (authenticity) to the bank note. Typical dimensions of a hot stamp thread are a width of 1-5 mm, a thickness of 3-4 .mu.m; windowed polyester terephthalate (PET) based threads have a thickness of about 0.5 mil or 12.5 microns. By way of example, one of the earliest forms of security threads consisted of reflective foil transferred by hot stamping to the surface the banknote (GB 2119312 A). This reflective foil prevented reproduction of counterfeit banknotes by printing processes such as from printing presses, PC printers and copiers. Holograms (EP-A-0624688), holographic features along with thermo chromic features (GB 2347646), opaque coatings having characters and patterns readable by transmitted light in combination with luminescent substances (U.S. Pat. No. 6,474,695), repeating patterns of magnetic/magnetic indicia or metal dots (WO02103624), laser etching fine lines and text with a laser (German “Auslegeschrift” no. 22 05 428) and (WO02101147), printing micro-characters on a metalized transparent plastic with clear acid resistant inks followed by acid etching of the unprinted areas to produce shiny micro-characters on a transparent base (U.S. Pat. No. 4,652,015), bonded nucleic acid molecules so that complementary nucleic acid molecules can bind to the molecules already attached to the document (DE 10122836), and optically variable security elements using liquid crystal material (EP0435029) have all been used to make security threads. However, these aforementioned optical device either take too much time to make and or have other associated problems; for example, it is found that laser etching takes too long to be cost effective, etching by use of chemicals requires multiple steps and is not considered to be environmentally-friendly; holograms can be readily copied, and in many instances the features of these security devices are not readily seen by eye by the average person and machines are required to read them.
A method to pattern a single layer of metal or carbon in a vacuum chamber was advanced in U.S. Pat. No. 4,022,928 by Piwcyzk. Piwcyzk used various methods to apply a perfluoropolyether known as FOMBLIN™ or Krytox™ to a substrate requiring a pattern for a vacuum deposited layer. The perfluoropolyether inhibited the deposition of the depositing material to a web or plastic substrate. Application of this fluid was by spray or vacuum evaporation in combination with a selected removal process as with a laser or an electron beam. A printing method was also described for applying the perfluoropolyether. Printing techniques including relief printing such as letterpress or flexography, planographic printing such as offset lithography, and gravure, and screen-printing such as silkscreen process printing were disclosed.
Subsequently, Ronchi in U.S. Pat. No. 4,749,591 incorporated herein by reference, and in PCT application WO 8700208(A1)) advanced this printing process by applying the inhibiting oil, FOMBLIN, to a vacuum roll coater where patterning thin films on plastic substrates was desired.
A major impediment to providing several thin film layers, was residual oil remaining on the images and on non-patterned areas of the web. This residual oil was detrimental to further thin film coating since left over oil would cause “ghosting”; a process whereby the inhibiting oil is transferred to the back side of plastic film when roll coating, which in turn causes inhibiting oil to be transferred further down the web on the front side. Left over inhibiting oil also causes adhesion failures to subsequent thin film layers.
In an effort to overcome impediments related to using inhibiting oil in providing a windowed image other techniques have been considered which use optically variable coatings.
Optically variable inks or coatings are composed of optically variable pigments, suspended in an ink, paint, or coating vehicle which is typically a polymer resin and may also contain other pigments, dyes, and additives. Optically variable pigments, such as the vacuum deposited optical multilayer pigments SecureShift™, Chromaflair™ and OVP™ pigments from JDSU Corporation, mica based pearlescent pigments such as those available from Englehard, Merck and others, and liquid crystal pigments are dependent for their effects on the layered structure and orientation of plate-like particles. For this reason, rather large platelets, typically ranging in size from a few micrometers to about 100 micrometers are preferred. If the particles become too small they fail to orient properly, and the brightness, purity, and degree of color or brightness change effects are reduced. The average size of such particles is typically larger than about 5 microns.
The platelet form of optically variable pigments results in difficulty in the production of fine features in printing processes. The optically variable particles themselves have dimensions much larger than those of conventional ink pigments which on average are smaller than 5 microns, and this leads to difficulty in dispersing the platelets and printing using conventional printing techniques. The printing of fine features requires that the pigment particles be significantly smaller than the feature size to be printed, so that the feature will appear to be continuous. This requirement is familiar from observation of displays composed of discrete elements, for example television and computer display screens, where picture features which approach the size of the display elements (pixels) become blurred and indistinct. There is a further problem with printing inks which have platelets larger than the desired feature size. Such platelets “bridge” across any closely spaced print regions of the printing plate, thus merging the regions in the printed article. Thus, even if the color shift areas are large compared to the platelet sizes, there can be no thin line boundaries between the color shift area and other printed features due to this bridging effect by the pigment particles.
One way in which these difficulties might be overcome, is to overprint fine contrasting and masking features on top of the optically variable ink features with a conventional fine particle ink. However, in practice it is not possible to get high print quality with this method, because the optically variable ink layer is thick, particularly in the case of magnetically oriented optically variable ink such as JDSU “Phantom™” ink, or mica-based pigmented inks, and the ink surface itself additionally may be quite rough due to the relatively large platelets embedded in the optically variable print region. Thus it is very difficult to overprint an optically variable ink pattern with a fine line closely controlled edge pattern. Close control of the placement, impression force, and consistency of ink application in a fine line pattern is not possible when printing on a non-planar surface.
The difficulties inherent in producing high resolution features using optically variable inks and coatings are overcome by the method of the present invention, in which the high resolution features are defined by printing with conventional inks, which comprise very fine or nanoparticulate pigments capable of printing high resolution features. The features are printed reversed on the substrate, leaving openings through which the optically variable component or layer may be viewed. Thus, it is only necessary for the optically variable layer to be printed behind the entire area which has openings for its viewing in the opaque print layer. In use, the article is viewed from the unprinted substrate side in the case of a transparent substrate, either as a label or printed article or as a hot stamp transfer to a receiving support article.
The optically variable component may be applied over the openings in the high resolution printed area either as an ink or coating, as optically variable pigment in an adhesive layer, or as a direct vacuum coated layer. Since the high resolution printed layer acts as a viewing mask when viewed from the substrate side, the optically variable component may be applied uniformly over the entire article, thus obviating the need for high resolution or fine features in the optically variable layer. Optionally, to conserve what may be costly optically variable ink or pigment, the optically variable component may be applied only to completely cover and overlap the open areas of the opaque ink mask.
Further, the application of the optically variable ink behind a conventional ink mask renders possible the production of individual items with unique content such as serial numbers, bar codes, images, and the like formed of optically variable effects by using for example inkjet, thermal transfer, or electrostatic printing methods to define the ink mask. Direct variable printing, especially at high resolution, is not practical with optically variable inks, due to their large particle size.
This invention provides security a decorative and/or security device which obviates the requirement of applying inhibiting oil and provides a simple means by which windows can be formed on a plastic substrate. In particular, a new optically variable security device having a high pattern resolution was made that contained readable text or graphic images where covert features could also be incorporated.
It is an object of this invention, to provide a security device having optically variable features such as an optically variable pattern that can be seen against a background that is distinguishable from the pattern, or from which the pattern stands out.
It is an object of this invention to provide a reverse printed image having gaps defined within the image defining windows, printed directly upon a light transmissive substrate, wherein gaps within the reverse printed image are coated thereover with a special effect coating of flakes, wherein the flakes can be seen through the substrate and wherein one of the reverse printed image and the flakes provide a background color for the other.
Special effect flakes include but are not limited to: color shifting flakes, color switching flakes, diffractive flakes; reflective flakes, covert flakes carrying covert information; purposefully shaped flakes, for example uniformly shaped flakes; magnetic flakes; magnetically alignable flakes, flakes containing fluorescent light emitting and/or wavelength conversion phosphors which respond to illumination at a first wavelength and emit energy at another wavelength, and/or combinations thereof. Special effect coatings are coatings comprised of a carrier having special effect flakes therein wherein the carrier in combination with the flakes may provide a special effect.